The Islamic State (IS) group released a graphic video yesterday in which black-clad militant Jihadi John claimed to have beheaded US aid worker Peter Kassig, who was captured last year.
It was released just hours after reports emerged that the terrorist, who is believed to be British, was injured in an allied air strike.
The masked fanatic, who speaks with a London accent, is believed to be responsible for the killings of David Haines and Alan Henning - both from Britain - and Americans James Foley, Steven Sotloff and now Mr Kassig.
Footage of Mr Kassig's death was posted online yesterday, but it is not known if it was shot before the reported US-led air strike on a summit of IS commanders.
It is claimed Jihadi John was taken to hospital after narrowly escaping death in the November 8 raid on the town of Al Qaim, near Iraq's border with Syria.
In yesterday's video, the militant was standing over a severed head, but it was not immediately possible to confirm that it was Mr Kassig (26).
The video, which was posted on websites used by IS in the past, appeared to be the latest in a series of blood-soaked messages to the US warning of further brutality if it does not abandon its air campaign in Iraq and Syria.
"This is Peter Edward Kassig, a US citizen, of your country - Peter who fought against the Muslims in Iraq while serving as a soldier," the militant says near the end of the nearly 16-minute video.
The video identifies the militant's location as Dabiq, a small town in the northern Syrian province of Aleppo, near the Turkish border.
It also shows what appears to be the mass beheading of several Syrian soldiers captured by the group. IS has warned that US soldiers will meet a similar fate.
"We say to you, Obama . . . you claim to have withdrawn from Iraq four years ago," the militant says.
"Here you are: you have not withdrawn. Here we are, burying the first American crusader in Dabiq, eagerly waiting for the remainder of your armies to arrive."
Mr Kassig, a former US Army Ranger, was providing medical aid to Syrians fleeing the civil war when he was captured inside Syria on October 1 last year. His friends say he converted to Islam in captivity and took the first name Abdul-Rahman.
Previous videos have shown the beheading of two American journalists and two British aid workers. The latest video did not show the person identified as Mr Kassig being beheaded.
Unlike previous videos, it did not show other western captives or directly threaten to behead anyone else.
The group also holds British photojournalist John Cantlie, who has been shown in several videos delivering long statements in English on the group's behalf, perhaps under duress.
Mr Kassig's family said in a statement that they were aware of the reports of the video and were awaiting confirmation from the US government.
"The family respectfully asks that the news media avoid playing into the hostage takers' hands and refrain from publishing or broadcasting photographs or video distributed by the hostage takers," they said.
"We prefer our son is written about and remembered for his important work and the love he shared with friends and family, not in the manner the hostage takers would use to manipulate Americans and further their cause."
Mr Kassig formed the aid organisation Special Emergency Response and Assistance, or SERA, in Turkey to provide aid and assistance to Syrian refugees. He is also a trained medical assistant who provided trauma care to wounded Syrian civilians.
IS has beheaded and shot dead hundreds of captives, mainly Syrian and Iraqi soldiers, during its sweep across the two countries, and has celebrated its mass killings in a series of extremely graphic videos.
The group has declared an Islamic caliphate in the areas under its control in Syria and Iraq, which it governs according to an extremely violent interpretation of Shariah law.
The US began launching air strikes in Iraq and Syria earlier this year in a bid to halt the group's rapid advance and eventually degrade and destroy it.